High co-expression of the ghrelin and LEAP2 receptor GHSR with pancreatic polypeptide in mouse and human islets

Islets represent an important site of direct action of the hormone ghrelin, with expression of the ghrelin receptor (growth hormone secretagogue receptor; GHSR) having been localized variably to alpha-cells, beta-cells, and/or somatostatin (SST)-secreting delta-cells. To our knowledge, GHSR expression by pancreatic polypeptide (PP)-expressing gamma-cells has not been specifically investigated. Here, histochemical analyses of Ghsr-IRES-Cre X Cre-dependent ROSA26-YFP reporter mice showed 85% of GHSR-expressing islet cells co-express PP, 50% co-express SST, and 47% co-express PP + SST. Analysis of single-cell transcriptomic data from mouse pancreas revealed 95% of Ghsr-expressing cells co-express Ppy, 100% co-express Sst, and 95% co-express Ppy + Sst. This expression was restricted to gamma-cell and delta-cell clusters. Analysis of several single-cell human pancreatic transcriptome datasets revealed 59% of GHSR-expressing cells co-express PPY, 95% co-express SST, and 57% co-express PPY + SST. This expression was prominent in delta-cell and beta-cell clusters, also occurring in other clusters including gamma-cells and alpha-cells. GHSR expression levels were upregulated by type 2 diabetes mellitus in beta-cells. In mice, plasma PP positively correlated with fat mass and with plasma levels of the endogenous GHSR antagonist/inverse agonist LEAP2. Plasma PP also elevated upon LEAP2 and synthetic GHSR antagonist administration. These data suggest that in addition to delta-cells, beta-cells, and alpha-cells, PP-expressing pancreatic cells likely represent important direct targets for LEAP2 and/or ghrelin in both mice and humans.

The SNAG Domain of Insm1 Regulates Pancreatic Endocrine Cell Differentiation and Represses Beta- to Delta-Cell Transdifferentiation

The allocation and specification of pancreatic endocrine lineages are tightly regulated by transcription factors. Disturbances in differentiation of these lineages contribute to the development of various metabolic diseases, including diabetes. The Insulinoma-associated protein 1 (<i>Insm1</i>), which encodes a protein containing one SNAG domain and five zinc fingers, plays essential roles in pancreatic endocrine cell differentiation and in mature beta-cell function. In the present study, we compared the differentiation of pancreatic endocrine cells between Insm1 null and Insm1 SNAG domain mutants (Insm1delSNAG) to explore the specific function of the SNAG domain of Insm1. We show that the delta-cell number is increased in Insm1delSNAG but not in Insm1 null mutants as compared to the control mice. We also show a less severe reduction of the beta-cell number in Insm1delSNAG as that in Insm1 null mutants. In addition, similar deficits are observed in alpha-, PP- and epsilon-cell in Insm1delSNAG and Insm1 null mutants. We further identified that the increased delta-cell number is due to beta- to delta-cell transdifferentiation. Mechanistically, the SNAG domain of Insm1 interacts with Lsd1, the demethylase of H3K4me1/2. Mutation in the SNAG domain of Insm1 results in impaired recruitment of Lsd1 and increased H3K4me1/2 levels at <i>H</i><i>hex</i> loci that are bound by Insm1, thereby promoting the transcriptional activity of the delta-cell-specific gene <i>Hhex</i>. Our study has identified a novel function of the SNAG domain of Insm1 in the regulation of pancreatic endocrine cells differentiation, particularly in the repression of beta- to delta-cell transdifferentiation.

The SNAG Domain of Insm1 Regulates Pancreatic Endocrine Cell Differentiation and Represses Beta- to Delta-Cell Transdifferentiation

The allocation and specification of pancreatic endocrine lineages are tightly regulated by transcription factors. Disturbances in differentiation of these lineages contribute to the development of various metabolic diseases, including diabetes. The Insulinoma-associated protein 1 (<i>Insm1</i>), which encodes a protein containing one SNAG domain and five zinc fingers, plays essential roles in pancreatic endocrine cell differentiation and in mature beta-cell function. In the present study, we compared the differentiation of pancreatic endocrine cells between Insm1 null and Insm1 SNAG domain mutants (Insm1delSNAG) to explore the specific function of the SNAG domain of Insm1. We show that the delta-cell number is increased in Insm1delSNAG but not in Insm1 null mutants as compared to the control mice. We also show a less severe reduction of the beta-cell number in Insm1delSNAG as that in Insm1 null mutants. In addition, similar deficits are observed in alpha-, PP- and epsilon-cell in Insm1delSNAG and Insm1 null mutants. We further identified that the increased delta-cell number is due to beta- to delta-cell transdifferentiation. Mechanistically, the SNAG domain of Insm1 interacts with Lsd1, the demethylase of H3K4me1/2. Mutation in the SNAG domain of Insm1 results in impaired recruitment of Lsd1 and increased H3K4me1/2 levels at <i>H</i><i>hex</i> loci that are bound by Insm1, thereby promoting the transcriptional activity of the delta-cell-specific gene <i>Hhex</i>. Our study has identified a novel function of the SNAG domain of Insm1 in the regulation of pancreatic endocrine cells differentiation, particularly in the repression of beta- to delta-cell transdifferentiation.

Innervation modulates the functional connectivity between pancreatic endocrine cells

Direct modulation of pancreatic endocrine cell activity by autonomic innervation has been debated. To resolve this question, we established an in vivo imaging model which also allows chronic and acute neuromodulation. Starting at a stage when zebrafish islet architecture is reminiscent of that in adult rodents, we imaged calcium dynamics simultaneously in multiple islet cell types. We first find that activity coupling between beta cells increases upon glucose exposure. Surprisingly, glucose exposure also increases alpha-alpha, alpha-beta and beta-delta coordination. We further show that both chronic and acute loss of nerve activity diminish activity coupling, as observed upon gap junction depletion. Notably, chronic loss of innervation severely disrupts delta cell activity, suggesting that delta cells receive innervation which coordinates its output. Overall, these data show that innervation plays a vital role in the establishment and maintenance of homotypic and heterotypic cellular connectivity in pancreatic islets, a process critical for islet function.